GB2240284A - Catalytic system and process for producing synthesis gas by reforming light hydrocarbons with CO2 - Google Patents

Abstract

A catalytic system for the production of synthesis gas by reacting light hydrocarbons, preferably methane, with CO2 is described formed from: - one or more compounds of metals of the platinum group, preferably chosen from rhodium, ruthenium and iridium; - a support consisting of inorganic compounds chosen from oxides and/or spinels of aluminium, magnesium, zirconium, silicon, cerium and/or lanthanum, possibly in the presence of alkaline metals, in which the weight percentage of the metal or metals of the platinum group in the catalytic system is between 0.01 and 20%, and preferably between 0.1 and 5%.

Description

CATALYTIC SYSTEM AND PBOCESS FOR PRODUCING SYNTHESIS GAS BY REFORMING LIGHT HYDROCARBONS WITH CO2 This invention relates to a catalytic system and its use in a reforming process for the single-stage production -of a gaseous mixture of Hz and CO.

The ain reactants used are CO2 and light hydrocarbons, preferably methane. The chemical equation describing the process of the present invention is: CO2 + CH4 =

2CO + 2H2 (1) Hydrocarbon reforiing reactions using CO2 have certain considerable advantages compared with the widespread steal reforaing processes described by the chemical equation: HzO + CH4

CO + 3Hz (2) Processes ainly using reaction (1) represent the best method for producing mixtures of H2 and CO if the natural gas used as the feedstock contains large CO2 quantities.

In addition an Hz/CO fixture in a ratio close to 1, as can be easily obtained by this invention, can be used advantageously in alcohol synthesis and in oxosynthesis. Currently, using syngas produced by the stead reforming reaction (2), the obtained H2/CO mixture has a ratio > 3. To obtain smaller ratios a second stage has to be used employing the reaction: COz + H2

CO + HzO (3) Adjusting the CO/Hz ratio by this chemical reaction negatively affects the overall economy of the process.

A potential use of a reforming process using CO2 as the lain reactant instead of steu is in Fischer-Tropsch synthesis plants in which the COz and methane produced could be again recycled to syngas of low H2/CO ratio.

The reforming process described by reaction (1), which produces H2/CO mixtures in an approximately equimolecolar ratio@ in a single stage, can also be advantageously used in highly integrated plants for ferrous mineral reduction.

Finally, methane reforiing processes employing CO2 can be advantageously used, compared with steal reforiing reactions, in thermal cycles for energy storage and transport by thermochemical pipe (TCP) [see T.A. Chubb, Solar Energy, 24, (1980) 341].

However, in contrast to steal reforming, system involving H2 and CO synthesis from CO2 and from light hydrocarbons do not have a well defined technology behind then The Ni-based catalysts usually used in steal reforming processes are not sufficiently selective, and deactivate rapidly when the H20/C ratio is less than 2 [see R.E.

Reitmeier, K. Atwood, H.A. Bennet Jr. and H.M. Baugh, Ind. Eng.

Chew. 40 (4), 620 (1948)).

This deactivation is due to the foliation of carbon, which covers the active metal centres during catalysis and accumulates in the catalyst pores, possibly causing fragmentation.

A catalytic system has now been found which produces synthesis gas (H2 and CO) by a light hydrocarbon reforming reaction without undergoing any discernible deactivation due to coke formation by the reactions: 2CO

Co: + C CH4

2H2 + C even if the H20/C ratio is distinctly favourable to such formation.

The catalytic system according to the present invention is characterised by being formed fro.: - one or ore compounds of petals of the platinum group, preferably chosen fro. rhodium, ruthenium and iridium; - a support consisting of inorganic coapounds chosen from oxides and/or spinels of aluminium, magnesium, zirconium, silicon, cerium and/or lanthanum, either alone or in mutual coibination and possibly in the presence of alkaline metals, in which the weight percentage of the metal or petals of the platinum group in the catalytic system is between 0.01 and 20X, and preferably between 0.1 and 5X The supports used can also consist of silicified aluminium, magnesium, cerium or lanthanum oxides.

The surface area of the catalysts used preferably varies between 1 and 400 ,2/g and lore preferably between 10 and 200 Z/g, while the pore volute preferably varies between 0.1 and 3 cc/g and nore preferably between 0.5 and 2 cc/g.

The catalytic system can be obtained either by impregnating the inorganic compounds with a solution of a salt of the metals of the platinum group followed by thermal drying and calcining, or by dispersing the inorganic compounds in an organic solvent, then reacting this in a carbon monoxide or inert atmosphere with solutions of compounds of the petals of the platinum group.

In this second case, the generally exotheriic reaction, which results in coloured reaction products, is followed by filtration, drying and calcining.

More particularly, the catalytic system in question can be prepared by heterogeneous solid-liquid reaction at a teuperature of between O'C and 150 C, and preferably between 20'C and 50 C, between compounds of the petals of the plating group dissolved in an organic solvent and the stated inorganic compounds dispersed in the same solvent.

Following this procedure, the petal quantity which fixes to the substrate is determined vainly by the cheiical properties of the inorganic oxide rather than by its porosity and surface area.

These latter are however important with respect to the integrity and stability of the catalyst during the reforiing reaction. In this respect, the accumulation of carbon in too siall pores leads to material fragmentation. A reduced support surface area also results in a lesser dispersion of petal and favours sinter phenomena with consequent catalyst deactivation.

The method for preparing supports consisting of silicified aluminium, agnesiua, cerium or lanthanua oxides consists essentially of a condensation reaction between the inorganic oxide (of aluminium, magnesium, cerium or lanthanua) and a silicon compound containing hydrolyzable organic groups, followed by removal of the unhydrolyzed organic residues by combustion or reaction in the presence of steam Using such silicification methods, materials can be obtained containing percentages of silicon varying between 0.5 and 15X and preferably between 1 and 5% by weight.

The present invention also provides a catalytic reforming process for light hydrocarbons, preferably methane, which enables mixtures molecular of Hz and CO to be obtained in / ratios varying between 0.6 and 6, and preferably between 0.8 and 3.

This catalytic process is characterised by conducting the reforaing preferably in a single stage using the aforedescribed catalytic system and operating at a temperature of between 350'C and 850'C, and preferably between 550 C and 750 C, at a pressure of between 0.5 and 50 atm, and preferably between 1 and 40 atm.

If methane is used, the required volumetric OO2/CH4 reactant ratio is between 0.5 and 15, and preferably between 0.8 and 10.

Under all these thermodynamic conditions the process can also be conducted in the presence of steam, if the particular application of the product synthesis gas requires it.

In this respect, it is necessary only to adjust the relative COz and HzO feed quantities to obtain a synthesis gas with any desired H2/CO ratio from 1 to 6.

Although the process is particularly suitable for methane reforming reactions, any other light hydrocarbon or mixture can be used in the process.

For example C1-C4 paraffins and olefines can be used by suitably choosing optimum temperature and pressure conditions and C02 ratios.

Any nstural gas containing hydrocarbon mixtures in which the methane content preferably exceeds 80X by. volume can be used.

Some examples are given below to better illustrate the invention, which however is not to be considered limited by them or to them.

EXAMPLE 1 Catalyst oreDaration The inorganic oxide used as the support was prepared by the following procedure.

A commercial magnesium oxide (supplied by Carlo Erba) with a surface area of 210 i;g was suspended under stirring in a tetraethylsilicate (TES) solution. The teiperature was maintained between 80'C and 90'C to favour evaporation of the ethanol formed by the condensation reactions. A dry gaseous nitrogen stream was fed into the reaction environment. Gas chroiatograph analysis of the exit gas showed the formation of ethanol.

The condensation reaction was considered at an end when ethanol was no longer detected in the exit gas stream. At this point the temperature was raised to 180 C to distil off the unreacted TES.

The unreacted ethoxy groups bonded to silicon atoms anchored to the solid inorganic support were then hydrolyzed by feeding a stream of nitrogen and steam at 200 C. Ethanol was also detected in the gas stream during this step. Infrared spectrum analysis on the material obtained'up to this point shows the presence of numerous hydroxyl bands which were not present in the starting material. The solid was then heated to 850'C (5C/min) and maintained at this temperature for 10 hours. After this treatment the surface area had reduced to 32m2/g, the silicon content being 1.5X by weight.Differential thermal, thermogravimetric and infrared spectroscopic analysis conducted during three cycles at temperatures between 25 C and 750'C showed no significant alteration in the physico-chemical properties of the silicified materials obtained. 50 g of silicified magnesium oxide were then suspended in 100 ml of 2-methylpentane in a nitrogen atmosphere.

A second solution of 50 ml of the same solvent containing 0.91 g of Rh4(CO)lz in a CO atmosphere was dripped rapidly into the silicified oxide suspension under stirring. The organic solution decolours rapidly passing from intense red to colourless, with the white solid simultaneously colouring. It is filtered in an inert atmosphere to obtain a material containing 1X by weight of Rh in r highly dispersed condition, as could be deduced from an analysis of the vibrational carbonyl bands of the surface complexes (see Figure 1 showing the diffused reflectance spectrum obtained on the pulverulent solid, in which the horizontal axis represents the wave number in cs-l and the vertical axis represents Kubelka Munk intensity units).The transformations of the surface complexes during thermal reduction with hydrogen in gaseous CH4 and Co: atmospheres were also studied by infrared spectroscopy. This resulted in a satisfactory understanding of surface nucleation phenomena, ensuring high reproducibility in material preparation.

Reforming reaction The reforming reaction was conducted in a fixed bed quartz reactor containing 3 cc of catalyst by feeding a gaseous equimolecular stream of CH4 and COz at a pressure of one atmosphere. In-line gas chromatograph analysis was carried out on the et gas stream starting from 300 C and continuing until 750 C. The gas hourly space velocity was maintained at 1000 (l/kg.h).

Fissure 2 shows the various experimental CH4 and CO2 conversion values at the various temperatures investigated (shown by black squares and dots respectively). The same figure also shows the theoretically calculated conversion values for the equilibrium system for the reactions: CO2 + CH4

2CO + 2H2 (A) Co: + Hz

CO + HzO The theoretical conversion of CO2 at equilibriu is shown by triangles, and that of CH4 by white squares.

From the results obtained it can be deduced that the catalyst is extremely active and enables conversions close to the thermodynamic equilibrium conversions to be obtained within the temperature range studied. The H20 percentage in the reaction product mixture is also close to the values calculated for the system (A) at equilibrium. Figure 3 shows the theoretically calculated variations in the concentrations of the gaseous species with temperature for the system under examination at a total pressure of 1 atm. The experimentally obtained gaseous species concentrations faithfully reproduce this pattern.

The H2/CO ratio within the range of 650-750 C was slightly less than 1.

Table 1 shows the results obtained for catalytic tests lasting 100 hours at 700'C conducted with the catalysts described in Examples 1-4 and 6, compared with the results obtained using a commercial steam reforming catalyst (Example 7) containing approximately 15.5X of Ni supported on a-alumina.

During the tests, catalysts comprising Rh deposited on silicified magnesium oxide proved to be extremely active in catalyzing the reactions of the system (A) but, surprisingly, not in catalyzing the carbon formation reactions even where these are favoured under these conditions.

The results of quantitative carbon analysis on the discharged catalysts are shown in Table 1. During the 100 hours the activity and selectivity of the catalytic system remained constant.

EXAMPLE 2 The catalyst synthesis procedure described in Example l ta5 repeated but using a solution containing 1.05 g of Ru3(CO)lz to obtain a solid containing 1X of Ru by weight.

The reforming reaction was conducted as in Example 1 feeding the same reactant mixture under the same pressure and spatial velocity conditions at temperatures of between 300 C and 750'C. Again in this case the CH4 and CO2 conversions are close to equilibrium values even if slightly less than those obtained in Example 1 (see Table 1).

EXAMPLES 3-4 In these cases the catalysts used contained the noble metals Rh (Example 3) or Ru (Example 4) and silicified alumina. This latter was prepared by condensing tetraethylsilicate with a gamma alumina supplied by AKZO in accordance with the procedure described in Example 1. The catalytic systems thus obtained proved to possess the same characteristics as those described in Examples 1 and 2, ie active in catalyzing the reactions of the system (A) within the entire temperature range investigated but inactive in catalyzing the reactions involved in the formation of carbon on the catalyst.

Table 1 also shows the results obtained during 100 hour catalytic tests in these two cases.

EXAMPLE 5 In this case the reactants were a gaseous stream of-C2Hs and COz in a 1/2 ratio. The catalytic tests were conducted at temperatures between 400'C and 700 C using the catalyst of Example at 700 C 2. The ethane conversion/during a catalytic test lasting 100 hours was found to be 100% and the COz conversion 98X The H2/CO ratio was 0.7. The methane in the exit gas was less than 3X.

EXAMPLE 6 In this example the catalyst synthesis procedure described in Example 1 was modified in that the noble metal was deposited on the silicified oxide by an impregnation reaction conducted by dripping an aqueous solution of Rh nitrate onto the silicified oxide until it was just soaked. The catalyst obtained in this manner contained 1X (wt/st) of Rh. In this case the catalytic tests conducted as in Examples 1-5 showed that the material modifies its characteristics during the first 10 hours of reaction at 700 C During this period the conversion values increase until they settle down at the values shown in Table 1.

However after the induction period the characteristics of the catalytic systems described in Examples 1-4 are again obtained in this case.

EXAMPLE 7 - Comparative Compared with Example 1 a commercial steam reforming catalyst consisting of about 15.5% by weight of Ni supported on alumina was used.

The results obtained are shown in Table 1.

TABLE 1 %conv CO2 % conv CH4 sel. H2/CO mg C/g cat * (mol/mol) (100 h) Ex.l 84.1 73.4 96.2 0.87 < 0.5 Ex.2 75.6 68.3 97.1 0.90 0.5 Ex.3 81.4 70.1 96.0 0.85 < 0.5 Ex.4 71.5 63.7 96.7 0.88 0.8 Ex.6 81.2 70.8 96.0 0.85 0.7 Ex.7 68.5 64.3 85.2 0.92 65.2 * (moles CO + H2)/(moles CO + 112 + H2O + C).100

Claims (17)

1. A catalytic system for the production of synthesis gas by reforming hydrocarbon with CO2, comprising: one or more compounds of a platinum group metal; and a support comprising an inorganic compound chosen from oxides and/or spinels of aluminium, magnesium, zirconium, silicon, cerium and/or lanthanum, either alone or in combination; in which the weight of the platinum group metal or metals in the catalytic system is from 0.01 to 20*.

2. A catalytic system as claimed in claim 1, wherein the weight percentage of the platinum group metal or metals is from 0.1 to 5%.

3. A catalytic system as claimed in claim 1 or 2, wherein the platinum group metal is rhodium, ruthenium or iridium.

4. A catalytic system as claimed in any of claims 1 to 3, having a surface area of from 1 to 400 m2/g and a pore volume of from 0.1 to 3 cc/g.

5. A catalytic system as claimed in claim 4, having a surface area of from 10 to 200 m2/g and a pore volume from 0.5 to 2 cc/g.

6. A catalytic system as claimed in any of claims 1 to 5, wherein the support comprises silicified aluminium, magnesium, cerium or lanthanum oxide.

7. A catalytic system as claimed in any of claims 1 to 6, further comprising one or more alkali metals or alkaline earth metals on the support.

8. A catalytic system as claimed in claim 12, substantially as hereinbefore described.

9. A process for producing synthesis gas by reforming hydrocarbon with CO2, wherein the reforming is carried out in the presence of a catalytic system as claimed in any of claims 1 to 8, at a temperature of from 350 C to 850"C and at a pressure of from 0.5 to 50 atm.

10. A process as claimed in claim 9, wherein the temperature is from 550 to 750 C and the pressure is from 1 to 40 atm.

11. A process as claimed in claim 9 or 10, wherein the reforming is conducted in a single stage.

12. A process as claimed in any of claims 9 to 11, wherein the volumetric CO2/hydrocarbon ratio is from 0.5 to 15.

13. A process as claimed in claim 12, wherein the volumetric C02/hydrocarbon ratio is from 0.8 to 10.

14. A process as claimed in any of claims 9 to 12, wherein steam is present to such an extent that te H2/CO ratio is from 1 to 6.

15. A process as claimed in claim 9, wherein the hydrocarbon is methane.

16. A process as claimed in claim 9, substantially as hereinbefore described.

17. Synthesis gas produced by a process as claimed in any of claims 9 to 16.